In recent years, a printing system called a POD (Print On Demand) system is in widespread use.
This system is utilized as a flexible system that can change contents of printing at any time for each page and further for each booklet, because the system is composed of combined image forming apparatus and a bookbinding apparatus in which bookbinding is carried out on the bookbinding apparatus while images are formed on sheets on the image forming apparatus.
Further, in this system, a bookbinding apparatus that makes a booklet by bundling sheets in a way of gluing by glues is also used as a device that conducts binding processing for a sheet bundle.
In the bookbinding apparatus that conducts binding processing for a sheet bundle by glues, a skillful technique and adjustment on a bookbinding process are necessary for making a high quality booklet that has sufficient booklet strength and is free from unevenness of glues formed on the outer side thereof by protrusion of glues and by uneven coating, and various improvements have been made.
Unexamined Japanese Patent Application Publication No. 2000-168265 discloses a technology to form a glue layer having a coating thickness corresponding to a thickness of a sheet bundle, by changing a height of a surface of revolution of a scrape roller corresponding to a thickness of a sheet bundle, in a coating device in which a coating thickness of glues can be adjusted by the scrape roller, after coating glues by the coating roller.
Unexamined Japanese Patent Application Publication No. 2006-346984 discloses a technology to improve finish on an outer surface of a glued cover while securing adhesive strength by coating twice through changes of a distance between a coating roller and a coated surface.
The booklet strength can be considered by dividing it mainly into resistance for spine breakage and adhesive strength, and if these two strengths can be secured sufficiently, sufficient booklet strength can be obtained.
Spine breakage is a phenomenon wherein spine SS is broken and bent when sheet bundle Sa is broken at its medium position as shown in FIG. 14(a), and resistance against force F1 that causes the spine breakage is resistance for spine breakage.
The adhesive strength is resistance against extracting force F2 when extracting force F2 for extracting one up to several sheets S from booklet S3 is applied on the booklet S3 as shown in FIG. 14(b).
The resistance for spine breakage is strengthened more mainly, as glue layer PS is made to be thicker. Namely, it is possible to enhance resistance for spine breakage by making a coating thickness to be thicker in coating glues.
The adhesive strength is enhanced mainly by increasing an amount of glues soaking into a space between sheet S and sheet S both constituting sheet bundle Sa and soaking into sheet S.
In the coating process, it is possible to cause glues to soak into a space between a sheet and a sheet and into a sheet itself by applying pressure on a glue layer before glues are solidified, in the course of conducting coating process or after coating, which increases the adhesive strength.
Though it is possible to increase the resistance for spine breakage and the adhesive strength by making a coating thickness of glues to be thicker and by applying pressure on a glue layer, glues protrude to both end portions of the sheet bundle Sa in the aforesaid occasion, and side glue SG is formed as shown in FIG. 14(c).
When a cover sheet is fitted, side glue SG causes unevenness of glues to be formed on a surface of the cover sheet, and an irregular side glue SG is formed on a border section between sheet bundle Sa and cover sheet S2 as shown in FIG. 14(c), thus, the side glue SG leads to quality loss of a booklet.
Only control of coating thickness as in Unexamined Japanese Patent Application Publication No. 2000-168265 cannot secure booklet strength and booklet quality sufficiently.
It is also impossible for coating twice as in Unexamined Japanese Patent Application Publication No. 2006-346984 to secure booklet strength and booklet quality sufficiently.